1. Field of the Invention
The invention relates to a system for avoiding congestion in a mobile communication system, and more particularly to such a system which avoids a common communication line between a mobile station and a base station controller from being congested due to an increase in traffic, and resultingly, avoids degradation in communication quality in a mobile station.
2. Description of the Related Art
According to the IS-95 type mobile communication system standardized by U.S. Telecommunications Industry Association (TIA), when a common communication line of an asynchronous transfer mode (ATM) type is used in communication between a base station and a base station controller, a mobile station is designed to be connected to a base station through a wireless channel, and a base station is designed to be connected to a base station controller, which is a host relative to the base station, through a common communication line.
Voice signals transmitted between a mobile station and a base station controller are encoded in four rates of FULL, xc2xd, xc2xc and xe2x85x9 in dependence on an amount of data included in voice signals in accordance with the variable rate voice signal encoding system defined in the above-mentioned IS-95 type mobile communication system. When a user makes communication, voice signals are encoded at a relatively high rate to produce a long voice frame, whereas when a user makes little communication, voice signals are encoded at a relatively low rate to produce a short voice frame.
The thus produced voice frames having a variable length are turned into ATM cells, which are transmitted between a base station and a base station controller through a common communication line.
When a mobile station is in soft-hand-off condition, that is, when a mobile station is in a condition to make communication with a plurality of base stations, a base station controller transmits the same voice frames in the form of ATM cells to all base stations to which a mobile station is connected, through each of down common communication lines. Voice signals transmitted from base stations to a mobile station are synthesized at the mobile station in wireless form.
Voice signals transmitted from a mobile station are turned into ATM cells in all base stations to which the mobile station is connected, and are transmitted to a base station controller through an up common communication line. Then, the ATM cells are turned back to voice frames in the base station controller.
When a base station controller receives voice frames having the same order numbers in the same mobile station, through a plurality of base stations, the base station selects a voice frame having best quality, and transmits the thus selected voice frame to a host apparatus.
However, if a common communication line becomes congested due to an increase in traffic, voice frames existing between a base station and a base station controller are all abandoned before being turned into ATM cells.
There have been suggested a lot of systems for avoiding congestion in a mobile communication system.
For instance, Japanese Unexamined Patent Publication No. 11-164346 has suggested a mobile communication system in which one of hard-hand-off and soft-hand-off is selected so that a ratio of mobile communication terminals carrying out hard-hand-off is increased in the mobile communication system, ensuring reduction in communication traffic between a wireless base station controller and a wireless base station.
Japanese Unexamined Patent Publication No. 11-275622 has suggested an apparatus for carrying out hand-off among base stations. The apparatus is equipped in a mobile communication exchange including a control signal line and a communication line both of which are connected to another mobile communication exchange. The apparatus is comprised of a first table in which a frequency at which the control signal line is used is stored, a second table in which a frequency at which the communication line is used is stored, means for finding a non-used line with reference to the first table, when requested by a base station to carry out hand-off, and finding a non-used line with reference to the second table, when a non-used line cannot be found with reference to the first table, and means for carrying out hand-off through the use of the thus found non-used line.
Japanese Unexamined Patent Publication No. 10-79985, which is based on U.S. Pat. No. 6,108,322 filed by Michael D. Cotzin on Jun. 28, 1996 and assigned to Motorola Incorporated, has suggested a method of monitoring a channel by means of a mobile unit in a communication system for transmitting data frame. The method includes the steps of monitoring a signal quality of a current traffic channel, judging whether the signal quality of a current traffic channel is within a predetermined level, ignoring data received by the current traffic channel during a time-slot assigned to the mobile unit for receiving data, and monitoring a control channel during a time-slot assigned to the mobile unit for receiving data.
Japanese Unexamined Patent Publication No. 10-145834 has suggested a method of carrying out soft-hand-off between a mobile station and a plurality of base stations in a mobile communication system including a mobile station and a base station to which the mobile station is connected, the method including the steps of connecting adjacent base stations to each other through a channel logically separated from a mobile exchange, and carrying out soft-hand-off control led by one of a plurality of the base stations, through the channel between the mobile station and a plurality of base stations.
Japanese Patent No. 2702437 (Japanese Unexamined Patent Publication No. 8-265818) has suggested a mobile communication system having a function of switching a communication channel. The mobile communication system includes a first memory storing a threshold value, a second memory storing a level of a received electric field in association with a base station which transmitted the electric field, and a comparator which, on receipt of a warning that a level of an electric field is degraded, compares the level stored in the second memory to the threshold value. If a level of the received electric field is greater than the threshold value, a request of measuring a level of an electric field is transmitted only to a base station associated with the level greater than the threshold value.
The above-mentioned conventional systems for avoiding congestion in a mobile communication system is accompanied with a problem that when an activity ratio of a common communication line between a base station and a base station controller is increased, and at last, becomes congested due to an increase in the number of mobile stations covered by a base station, voice frames existing between mobile stations using the common communication line and the base station controller are all abandoned.
The conventional systems are accompanied further with a problem that when voice frames of a mobile station which is in a non soft-hand-off condition and which is connected to a base station controller through a common communication line which is a sole communication line for the mobile station, are abandoned, voice data lacks between the mobile station and a base station controller, resulting in degradation in communication quality.
In view of the above-mentioned problems in the conventional systems, it is an object of the present invention to provide a system for avoiding congestion in mobile communication which system is capable of reducing traffic in a common communication line and preventing degradation in communication quality in a mobile station using the common communication line as a sole communication line therefor.
In one aspect of the present invention, there is provided a system for avoiding congestion in mobile terminal communication, including (a) a plurality of mobile stations, (b) a plurality of base stations, and (c) a base station controller connected to the base stations through a common communication line, each of the mobile stations having a soft-hand-off function by which each of the mobile stations can make communication with the base stations, and having a function of encoding voice signals transmitted to and received from the base station controller, with a variable rate, the base station controller (a) monitoring an activity ratio of a down common communication line, (b) when it is judged that the activity ratio of a down common communication line may be congested, assigning an order of precedence to each of the mobile stations using the common communication line, based on the number of base stations to which each of the mobile stations concurrently makes communication, and (c) abandoning low-rate voice frames at a constant ratio among voice frames of a mobile station to which a low order of precedence is assigned.
For instance, the base station controller may be designed, when the activity rate increases, to gradually increase the ratio at which voice frames are abandoned among all voice frames, the rate of the voice frames, and the order of precedence of a mobile station transmitting and receiving voice frames to be abandoned.
It is preferable that the base station controller includes a first memory to store therein a first list of base stations to which each of the mobile stations using the common communication line concurrently makes communications, in which case, if there is a change in base stations to which each of the mobile stations concurrently makes communications, the first memory reflects the change to the list.
It is preferable that each of the base stations includes a second memory to store therein a first list of base stations to which each of the mobile stations using the common communication line concurrently makes communications, and wherein if there is a change in base stations to which each of the mobile stations concurrently makes communications, the first memory reflects the change to the list.
It is preferable that the base station controller includes a second list in which a rate of voice frames to be abandoned when the activity ratio of the common communication line reaches a threshold activity ratio, a ratio at which voice frames are to be abandoned among all voice frames, and the minimum number of base stations to which a target mobile station is connected are defined for each of activity ratios.
It is preferable that each of the base stations includes a second list in which a rate of voice frames to be abandoned when the activity ratio of the common communication line reaches a threshold activity ratio, a ratio at which voice frames are to be abandoned among all voice frames, and the minimum number of base stations to which a target mobile station is connected are defined for each of activity ratios.
It is preferable that the base station controller, when the activity ratio of the down common communication line reaches the threshold activity ratio, abandons voice frames at the rate and at the ratio without transmitting to the down common communication line in a mobile station or mobile stations using the down common communication line and connected to the base stations in a number equal to or greater than the minimum number of base stations.
It is preferable that each of the base stations, when the activity ratio of the up common communication line reaches the threshold activity ratio, abandons voice frames at the rate and at the ratio without transmitting to the up common communication line in a mobile station or mobile stations using the up common communication line and connected to the base stations in a number equal to or greater than the minimum number of base stations.
There is further provided a system for avoiding congestion in mobile terminal communication, including (a) a plurality of mobile stations, (b) a plurality of base stations, and (c) a base station controller connected to the base stations through a common communication line, each of the mobile stations having a soft-hand-off function by which each of the mobile stations can make communication with the base stations, and having a function of encoding voice signals transmitted to and received from the base station controller, with a variable rate, each of the base stations (a) monitoring an activity ratio of an up common communication line, (b) when it is judged that the activity ratio of an up common communication line may be congested, assigning an order of precedence to each of the mobile stations using the common communication line, based on the number of base stations to which each of the mobile stations concurrently makes communication, and (c) abandoning low-rate voice frames at a constant ratio among voice frames of a mobile station to which a low order of precedence is assigned.
In another aspect of the present invention, there is provided a method of avoiding congestion in a mobile terminal communication system including: (a) a plurality of mobile stations, (b) a plurality of base stations, and (c) a base station controller connected to the base stations through a common communication line, each of the mobile stations having a soft-hand-off function by which each of the mobile stations can make communication with the base stations, and having a function of encoding voice signals transmitted to and received from the base station controller, with a variable rate, the method including the steps of (a) monitoring an activity ratio of a down common communication line, (b) when it is judged that the activity ratio of a down common communication line may be congested, assigning an order of precedence to each of the mobile stations using the common communication line, based on the number of base stations to which each of the mobile stations concurrently makes communication, and (c) abandoning low-rate voice frames at a constant ratio among voice frames of a mobile station to which a low order of precedence is assigned, the steps (a), (b) and (c) being to be carried out by the base station controller.
The method may further include the step of, when the activity rate increases, (d) gradually increasing the ratio at which voice frames are abandoned among all voice frames, the rate of the voice frames, and the order of precedence of a mobile station transmitting and receiving voice frames to be abandoned, the step (d) being to be carried out by the base station controller.
The method may further include the steps of (e) storing therein a first list of base stations to which each of the mobile stations using the common communication line concurrently makes communications, and (f) if there is a change in base stations to which each of the mobile stations concurrently makes communications, reflecting the change to the list.
For instance, the steps (e) and (f) are to be carried out by the base station controller, or by each of the base stations.
The method may further include the step of (g) storing a second list in which a rate of voice frames to be abandoned when the activity ratio of the common communication line reaches a threshold activity ratio, a ratio at which voice frames are to be abandoned among all voice frames, and the minimum number of base stations to which a target mobile station is connected are defined for each of activity ratios.
For instance, the step (g) is to be carried out by the base station controller, or by each of the base stations.
The method may further include the step of, when the activity ratio of the down common communication line reaches the threshold activity ratio, (h) abandoning voice frames at the rate and at the ratio without transmitting to the down common communication line in a mobile station or mobile stations using the down common communication line and connected to the base stations in a number equal to or greater than the minimum number of base stations, the step (h) being to be carried out by the base station controller.
The method may further include the step of, when the activity ratio of the up common communication line reaches the threshold activity ratio, (i) abandoning voice frames at the rate and at the ratio without transmitting to the up common communication line in a mobile station or mobile stations using the up common communication line and connected to the base stations in a number equal to or greater than the minimum number of base stations, the step (i) being to be carried out by each of the base stations.
There is further provided a method of avoiding congestion in a mobile terminal communication system including: (a) a plurality of mobile stations, (b) a plurality of base stations, and (c) a base station controller connected to the base stations through a common communication line, each of the mobile stations having a soft-hand-off function by which each of the mobile stations can make communication with the base stations, and having a function of encoding voice signals transmitted to and received from the base station controller, with a variable rate, the method including the steps of (a) monitoring an activity ratio of an up common communication line, (b) when it is judged that the activity ratio of an up common communication line may be congested, assigning an order of precedence to each of the mobile stations using the common communication line, based on the number of base stations to which each of the mobile stations concurrently makes communication, and (c) abandoning low-rate voice frames at a constant ratio among voice frames of a mobile station to which a low order of precedence is assigned, the steps (a), (b) and (c) being to be carried out by each of the base stations.
In still another aspect of the present invention, there is provided a recording medium readable by a computer, storing a program therein for causing a computer to operate a base station controller in the above-mentioned system.
There is further provided a recording medium readable by a computer, storing a program therein for causing a computer to carry out the above-mentioned method.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the present invention, if either a base station or a base station controller judges that a common communication line connecting the base station and the base station controller to each other may be congested due to an increase in traffic, low-rate voice frames of a mobile station which uses a plurality of common communication lines and which is in a soft-hand-off condition are first abandoned. Hence, it would be possible to reduce traffic in a common communication line and prevent degradation in communication quality in a mobile communication using a common communication line as a sole communication line therefor.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.